The Hall-Héroult cell revolutionized aluminum production in 1886 (U.S. Pat. No. 400,664; herein incorporated by reference in its entirety) by reducing aluminum oxide dissolved in a molten salt, with a consumable carbon anode that reacts with the oxygen to form carbon dioxide. This type of electrolytic cell has been used more recently to produce other metals such as neodymium. Following invention of the Hall-Héroult cell, the aluminum industry and others have been seeking a material to serve as an inert anode in place of the carbon anode. The demands on such a material are very high: it must conduct electrons well, at high temperature, in direct contact with both oxygen and molten salt, both of which are at unit activity, and must not impede oxygen gas mass transfer. Most metals oxidize and/or evaporate in oxygen at high temperatures; most oxide conductors dissolve in the molten salt; and most other materials do not conduct electricity well enough for this application.
Use of a solid electrolyte, such as stabilized zirconia, between the molten salt and anode removes the anode requirement of chemical stability in contact with a molten salt. For reactive metals such as aluminum, magnesium, calcium and rare earth metals, the solid electrolyte improves current efficiency considerably by presenting a solid barrier between the metal produced at the cathode and oxidizing gases produced at the anode, preventing back-reaction (see, for example, U.S. Pat. Nos. 5,976,345 and 6,299,742; each herein incorporated by reference in its entirety). The process comprises a solid oxygen ion-conducting membrane (SOM) typically consisting of zirconia stabilized by yttria (YSZ) or other low valence oxide-stabilized zirconia, for example, magnesia- or calcia-stabilized zirconia (MSZ or CSZ, respectively) in contact with the molten salt electrolyte bath in which the metal oxide is dissolved, an anode in ion-conducting contact with the solid oxygen ion-conducting membrane, and a power supply for establishing a potential between the cathode and anode. The SOM process runs at high temperature, typically 1000-1300° C., in order to maintain high ionic conductivity of the SOM. However, an inert anode must still have high conductivity and stability in oxygen at high temperature.
Liquid silver and gold satisfy all of these requirements, and zirconia can serve as a container for such a liquid metal anode. This gives a minimum operating temperature of the silver-oxygen eutectic at 939° C. (J. Phase Equillibria 1992, 13(2), 137-142; hereby incorporated by reference herein in its entirety). Their alloys with other electronegative elements including, but not limited to, copper, tin, lead, bismuth, or combinations thereof can also satisfy these requirements at lower cost and lower temperature. However, silver and gold are very expensive, and any significant dilution with these alloying elements risks their oxidation. Even partial oxidation of the alloying elements would raise the alloy's solidus and liquidus temperatures and present a barrier to oxygen transport, as well as a possible corrodant to the zirconia electrolyte.
Thus, there remains a need for more stable and inexpensive anode systems to stabilize anodes in an oxygen generating environment. This invention addresses these needs.